1. Field of the Invention
The present invention relates to a printing technique that creates dots on a printing medium by forward and backward passes of main scan.
2. Description of the Related Art
The ink jet printer that causes a plurality of different color inks to be ejected from a print head is used as one of the output devices of the computer. For the enhanced printing speed, the multi-nozzle configuration and the bi-directional printing technique are generally applied for the ink jet printer. The multi-nozzle configuration uses a print head with a large number of nozzles arranged thereon, and the bi-directional printing technique carries out printing in both forward and backward passes of main scan.
The misalignment of positions where dots are created undesirably lowers the picture quality of the resulting image in the ink jet printer. It is thought that the positional misalignment is mainly ascribed to the ink ejection characteristics from the nozzles. The interlace technique or the overlap technique has been applied to reduce the effects of the positional misalignment and improve the picture quality of the resulting image.
The interlace technique forms raster lines in an intermittent manner in a sub-scanning direction to complete an image. For example, in the multi-nozzle configuration, it is assumed that the respective nozzles are disposed at intervals of not less than 2 raster lines in the sub-scanning direction. Because of this nozzle interval, every pass of the main scan forms raster lines in an intermittent manner in the sub-scanning direction. The vacant space between the existing raster lines is successively occupied by subsequent passes of the main scan with movements of the print head in sub-scan. The feeding amount of sub-scan is adequately selected to fill the vacancy and form adjoining raster lines with different nozzles. The interlace technique effectively disperses the positional misalignment in the sub-scanning direction, thereby making the positional misalignment sufficiently inconspicuous.
The overlap technique records dots on each raster line with at least two different nozzles. For example, the procedure creates dots in odd pixels on a certain raster line with one nozzle by a first pass of the main scan and dots in even pixels on the certain raster line with another nozzle by a second pass of the main scan. The overlap technique enables the positional misalignment to be well dispersed on the respective raster lines in a main scanning direction.
The interlace technique and the overlap technique are applicable for bi-directional printing. Dots created by the forward and the backward scans of the print head are arranged in various different patterns corresponding to the combinations of the nozzle pitch, the feeding amount of sub-scan, and the number of repeated scans.
In some cases, however, application of the interlace technique or the overlap technique does not sufficiently improve the picture quality. In the ink jet printers, a recent trend is the enhanced scanning speed of the print head to attain the higher-speed printing. Another trend is the use of finer dots for the better picture quality. Such attempts may, however, lead to the insufficient improvement in picture quality even under the application of the interlace technique or the overlap technique. This drawback is not negligible in the latest high-performance printers.
The object of the present invention is thus to improve the printing quality in the case of bi-directional printing.
At least part of the above and the other related objects is attained by the technique of the present invention, which is favorably applied to a printing apparatus where a partial area of main scan has a lower positional accuracy of dot creation. In such a printing apparatus, the technique records dots in such a manner that forward pass dots created in a forward pass of the main scan and backward pass dots created in a backward pass of the main scan are present at substantially equal rates in at least the partial area of the lower positional accuracy.
The inventors of the present invention have found that the positional misalignment of dot creation is ascribed to the mechanical configuration of the printing apparatus as well as to ink ejection characteristics. The inventors have also found that the positional misalignment appears at different positions in the forward pass and in the backward pass of the main scan. The technique of the present invention takes advantage of these findings and causes dots created in the forward pass to be mixed with dots created in the backward pass in an area of potential positional misalignment. This arrangement reduces the conspicuousness of the positional misalignment and thereby improves the picture quality of the resulting printed image.
Such a positional misalignment of dot creation may be ascribed to the mechanical vibrations of the print head in the course of the main scan. The increase in main scanning speed of the print head to enhance the printing speed and improve the performance of the printer leads to the greater mechanical vibrations of the print head. In general, the mechanical vibrations of the print head arise in the respective initial stages of the forward pass and the backward pass of the main scan (that is, at the time of accelerating the main scan) and gradually attenuate. In addition to the mechanical vibrations, application of the very fine dots created on the printing medium for the purpose of the improved printing quality causes even a slight positional misalignment of dot creation to make dropouts. In the case of bi-directional printing, the positional misalignment of dot creation is significantly conspicuous in some recording procedures.
FIG. 1 shows the effects of the mechanical vibrations on the picture quality. The dotted lines extending in the main scanning direction in images PICa and PICb represent raster lines. In the drawing of FIG. 1, the direction from left to right represents the direction of the forward pass of the main scan of the print head. The direction from right to left represents the direction of the backward pass of the main scan. As clearly understood from the illustration, the positions of dots are misaligned due to the mechanical vibrations of the print head arising in the respective initial stages of the forward pass and the backward pass of the main scan. The hatched areas represent specific areas of low positional accuracy, where the mechanical vibrations of the print head cause the positional misalignment of dot creation.
The upper portion of FIG. 1 shows an image PICa printed over the whole width in the main scanning direction. The image PICa has the specific areas of low positional accuracy on both ends thereof, due to the mechanical vibrations of the print head arising in both ends of the main scan range (that is, in the respective initial stages of the forward pass and the backward pass of the main scan).
There is a case in which an image occupies only part of the width in the main scanning direction. In such cases, the main scan is carried out only in the part of the width with the image, for the enhanced printing speed. The lower portion of FIG. 1 shows an image PICb printed by carrying out the main scan only in the part of the width in the main scanning direction. The image PICb also has the specific areas of low positional accuracy on both ends thereof.
In such areas of low positional accuracy, the positional misalignment of dot creation may be recognized visually as deterioration of the printing quality. FIG. 9 shows an arrangement, in which the bundle of 3 raster lines formed in the forward pass of the main scan and the bundle of 3 raster lines formed in the backward pass of the main scan appear alternately. In this drawing, the open circles represent the forward pass dots and the closed circles represent the backward pass dots. The dotted lines represent raster lines where dots are to be created. In this example, the effects of the positional misalignment of dot creation are observed on boundaries g1 through g3.
In one embodiment of the present invention, the recording procedure makes dots created in the initial stage of the forward pass of the main scan mixed with dots created in the terminal stage of the backward pass of the main scan in one end area of the main scan, while making dots created in the initial stage of the backward pass of the main scan mixed with dots created in the terminal stage of the forward pass of the main scan in the opposite end area of the main scan. In the respective initial stages of the forward pass and the backward pass of the main scan, the print head has large mechanical vibrations and creates dots with the poor positional accuracy. In the respective terminal stages, on the other hand, the vibrations sufficiently attenuate and resulting dots have the sufficiently high positional accuracy. This arrangement enables the dots of the poor positional accuracy and the dots of the sufficiently high positional accuracy to be present at substantially same rates in the areas of the lower positional accuracy. This effectively reduces the conspicuousness of the positional misalignment of dot creation and improves the printing quality in the case of bi-directional printing.
The technique of the present invention, which makes the dots of the relatively poor positional accuracy mixed with the dots of the relatively high positional accuracy, is applicable to the structure where the mechanical vibrations of the print head continue over the whole range of the main scan. The technique is also applicable to the structure where the mechanical vibrations of the print head do not significantly affect the printing quality.
In the technique of the present invention, the forward pass dots and the backward pass dots may be arranged to be adjacent to each other on an identical raster line or may be arranged alternately in the sub-scanning direction.
The technique of the present invention is favorably applied for a print head having a nozzle pitch of not less than 3 dot pitch in the sub-scanning direction.
The latest ink jet printers have extremely high resolutions and use very fine dots. The greater nozzle pitch of the print head is desirable from the viewpoints of the restrictions on manufacture of the print head and no overlap of blotting dots. In the case of the print head having the nozzle pitch equal to 2 dot pitch, there are only three procedures applicable to record dots: i) the first procedure creates dots in odd pixel (pixels having odd ordinal numbers allocated thereto) on each raster line in the forward pass of the main scan and dots in even pixels (pixels having even ordinal numbers allocated thereto) on the raster line in the backward pass of the main scan; ii) the second procedure forms odd raster lines (raster lines having odd ordinal numbers allocated thereto) in the forward pass of the main scan and even raster lines (raster lines having even ordinal numbers allocated thereto) in the backward pass of the main scan; and iii) the third procedure creates dots in pixels arranged in a checker pattern in one identical pass of the main scan. In the case of the print head having the nozzle pitch of not less than 3 dot pitch, however, there is a diversity of recording procedures having, for example, different feeding amounts of sub-scan and different numbers of repeated scans. The technique of the present invention specifies the optimum recording procedure to attain the improvement in picture quality, among the diversity of recording procedures.
In another embodiment of the present invention, the recording procedure makes the forward pass dots mixed with the backward pass dots in response to the setting of a predetermined print mode. This procedure of the embodiment changes the print data to be generated according to the selected print mode. In one possible modification, the print data to be generated may be changed according to the type of the printing medium. In another possible modification, the print data may be changed, based on the printing quality of the actual printing result. The predetermined print mode may be set arbitrarily and is, for example, a print mode used for printing natural images. In general, the natural image includes a large number of image parts in a medium tone range, where the positional misalignment of dots is rather conspicuous compared with image parts in a low tone range or those in a high tone range. Here the medium tone range represents tones in the middle of a reproducible tone range of the printing apparatus. Application of the recording method of the present invention for the natural images including lots of medium tones effectively improves the printing quality in the case of bi-directional printing.
The technique of the present invention is preferably applied for a print head that creates dots on a printing medium in such a manner that the dot percent is not greater than a predetermined level corresponding to the printing resolution. In general, the diameter of dots used for printing is set according to the resolution of the printed image. The dot diameter is affected not only by the printing resolution but by the type of the printing medium, the type of inks, and the printing environment. For example, the diameter of dots formed on special paper is smaller than the diameter of dots formed on ordinary paper. Even a slight positional misalignment of dot creation is likely to cause dropouts between such small-diametral dots adjoining to each other. This leads to deterioration of the printing quality. The small diametral dots are also affected significantly by the mechanical vibrations of the print head arising in the course of the main scan. The technique of the present invention is thus especially effective in the structure using the dots of a relatively small diameter.
The technique of the present invention is preferably applied for a print head that creates dots with pigment inks. In the case of printing with ordinary dye inks, ink droplets hit against the printing medium form adequately blotting dots. In the case of printing with pigment inks, however, the resulting dots hardly blot. Compared with printing with the ordinary dye inks, printing with the pigment inks is more likely to make dropouts, which have no dots formed therein, due to the positional misalignment of dot creation. This leads to deterioration of the printing quality. Application of the technique of the present invention to the printing apparatus using the pigment inks desirably improves the printing quality in the case of bi-directional printing.
The technique of the present invention is attained by a diversity of applications, which include a printing apparatus and a print control apparatus that generates print data for controlling an operation of a printer unit, as well as a printing method and a printing control method corresponding thereto. The technique may also be attained by a computer program to actualize any of such apparatuses and methods, a recording medium in which such a computer program is recorded, and a data signal that includes such a computer program and is embodied in a carrier wave.
In the case where the technique of the present invention is constructed in the form of a computer program or a recording medium in which the computer program is recorded, it may include all the functions of the print control apparatus or the printing apparatus or alternatively only part of the functions that are characteristic of the present invention. Typical examples of the recording medium include flexible disks, CD-ROMs, magneto-optic discs, IC cards, ROM cartridges, punched cards, prints with barcodes or other codes printed thereon, internal storage devices (memories like a RAM and a ROM) and external storage devices of the computer, and a variety of other computer readable media.